Diamond has numerous outstanding properties not seen in other semiconductor materials, such as its high thermal conductivity, high electron/hole mobility, high dielectric breakdown field, low dielectric loss, and wide bandgap. In particular, recent years have witnessed the continued development of ultraviolet emitting elements that take advantage of wide bandgap, as well as field effect transistors and the like having excellent high frequency characteristics.
Manmade diamond monocrystals, which are usually produced by a high-temperature, high-pressure synthesis process, have excellent crystallinity and, because of a phonon-related thermal conduction mechanism that is different from that of metals, have a thermal conductivity that is at least 5 times that of copper at normal temperature. These features are put to use when these diamond monocrystals are used as a heat-spreading substrate that needs high performance and reliability. In contrast, with a diamond polycrystalline film, which is usually obtained by vapor phase synthesis, because of the effect of phonon scattering at the grain boundary, the thermal conductivity is only about half that of diamond monocrystals.
Meanwhile, a large diamond composite substrate is needed in order for diamond to be used in semiconductor applications. Because they have better crystallinity than naturally occurring monocrystals, diamond monocrystals obtained by a high-temperature, high-pressure process are also useful as semiconductor substrates. However, the ultra-high-pressure synthesis apparatus used in this high-temperature, high-pressure process is bulky and expensive, which means that there is a limit to how much the cost of manufacturing monocrystals can be reduced. Also, since the size of the resulting monocrystals is proportional to the apparatus size, a size on the order of 1 cm is the practical limit. In view of this, Japanese Patent Publication H3-75298 (Patent Document 1), for example, discloses a method for obtaining a diamond monocrystalline substrate that has a large surface area. In the method, a plurality of high-pressure phase substances having substantially the same crystal orientation are arranged, a substrate that will serve as a nucleus for vapor phase growth is formed, and monocrystals are grown over this by vapor phase synthesis, resulting in integrated, large monocrystals.
Japanese Patent Publication H2-51413 discusses a method in which at least two diamond surfaces are provided with a space in between, and then diamond or diamond-like crosslinks are grown between the diamond surfaces by chemical vapor deposition (CVD), thereby joining the diamond surfaces. Nevertheless, this joined diamond that has been crosslinked between two surfaces has a drawback in that when the surface is polished, polishing stress is concentrated at the joint interface, leading to separation at the joint.
Problems encountered when diamond monocrystals are used as a heat-spreading substrate are thermal strain and cracking, which are attributable to the difference in the coefficients of thermal expansion between diamond and the heat emitting material. Diamond has one of the lowest coefficients of thermal expansion of all substances, whereas semiconductor materials such as silicon and GaAs have coefficients of thermal expansion from 1.5 to several times that of diamond, so when the two are heated and soldered, for example, to join them, deformation and cracking occur during cooling. In particular, diamond monocrystals have a large Young's modulus and are resistant to deformation, which conversely makes them a brittle material with low toughness. Specifically, the drawback to diamond monocrystals is that when they are subjected to a force, they tend to cleave along their {111} plane. Accordingly, there has also been practical application of heat-spreading substrates that feature polycrystalline diamond, which has higher toughness than monocrystals. Still, the thermal conductivity of monocrystals cannot be matched by the above-mentioned diamond polycrystalline film alone.
The inventors conducted the method for obtaining large monocrystals discussed in the above-mentioned Patent Document 1 in order to examine any problems encountered with this method, and found that the following problem occurs. A monocrystalline substrate consisting of a plurality of layers serving as nuclei for vapor phase growth usually does not have exactly the same orientation of the growth planes, which each layer having a slightly different planar orientation. When monocrystalline vapor phase growth is conducted in this state and the monocrystals are integrated, the joined portions thereof have growth boundaries of different angles, called small angle boundaries, which are defects in the broad sense, and these basically do not disappear no matter how long the monocrystalline growth is continued.
The inventors used a Raman scattering spectroscope to examine in detail the area around this small angle boundary, and as a result measured a peak shift that is different from that of an ordinary diamond peak. Specifically, what they found was not the ordinary diamond monocrystal peak near 1332 cm−1, but rather the presence of a micro-region shifted a few cm−1 higher or lower in the vicinity of the monocrystal connection boundary. They also found that when monocrystalline growth was continued in this same state, the monocrystals came apart during vapor phase growth around the monocrystal connection boundary at roughly the point when the film thickness exceeded 100 μm. From these two facts they recognized a problem in that when large monocrystals are formed by the above-mentioned prior art, stress accumulates near the small angle boundaries, and the monocrystals come apart around these boundaries at or over a certain film thickness.